1. Field of the Invention
The present invention relates to the field of semiconductor amplifiers and in particular to an amplifier configuration in which a pair of cascaded amplifiers are required to operate in alternation, and to achieve high current gain when "on" and high inverse current when "off". The application of the present amplifier is to an automatic gray scale control circuit for a color television receiver.
The invention also relates to the field of semiconductor fabrication, and more particularly to a monolithic semiconductor structure suitable for moderately high voltage applications.
2. Description of the Prior Art
The advent of integrated circuits has given the designer a powerful tool in assembling collections of transistors. However, the technology as it is practical today is economically feasible only at low voltages and with small power dissipations. Thus, for high voltage applications, one is forced to use fabrication techniques suitable for individual transistors. In the event that multiple transistor functions are desired, it may often be economical to form unitary assemblies of a relatively small number of transistors. One such assembly is disclosed in U.S. Pat. No. 2,663,806 to Darlington, entitled "Semiconductor Signal Translating Device".
The Darlington patent cited above describes a two or three transistor structure wherein successive transistors are connected in cascade. Cascading is required in the present application to achieve adequate gain and two alternately active cascaded amplifiers are required. In addition, the control application herein contemplated makes it desirable that the input and output functions share common terminals and that a given cascaded amplifier pair be characterized by a high inverse impedance when in the "off" state.
The contemplated application is to an automatic gray scale control circuit for a color television receiver. In the foregoing application the voltages to which the transistors are exposed are on the order of two hundred volts, and the leakage currents when a cascaded amplifier is turned off should be substantially less than the functional microampere currents of the cathode ray tube, when it is self-biased to cut-off. In this application, the function of the semiconductor amplification means is to sense the voltage at the cathode of a gun of a color cathode ray tube, when it is cut off during blanking and charge a capacitor to that voltage. During the video field, the driver and cathode are supplied current at the same terminal used for sensing the gain cut off potential at the voltage stored on the capacitor.